Polycarbonates obtained from 2,2-bis(4-hydroxyphenyl)propane (hereinafter abbreviated bisphenol A) and a carbonate precursor substance, such as diphenyl carbonate or phosgene, are conventionally known as typical aromatic polycarbonates. Since they have a variety of excellent properties; that is, they are transparent and they are excellent in heat resistance and mechanical properties, and good in dimensional accuracy, they are widely used as engineering plastics. However, in recent years, amid the trend that light weight, thinness, and compactness (downsizing) of machinery, tools, and the like are considered important, there are increased cases in which engineering plastics are used at locations closer to a heat source in optical usage. Consequently, it is required that engineering plastics be good in optical properties, such as light transmittance, and in addition higher in hydrolysis resistance, heat resistance, heat stability, and oxidation resistance.
On the other hand, it is known that aromatic polycarbonates excellent in heat resistance can be obtained by reacting 2,2-bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane (hereinafter abbreviated to bisphenol AF) with a carbonate precursor substance (Japanese Patent Publication No. 12283/1991). However, aromatic polycarbonates obtained from usual bisphenol AF are poorer in hydrolysis resistance and heat stability than polycarbonates from bisphenol A; in addition the glass transition temperature thereof is less than 160.degree. C., and the heat resistance thereof is not sufficiently satisfactory. It is also known that an aromatic polycarbonate excellent in heat resistance is obtained by reacting 9,9-bis(4-hydroxyphenyl)fluorene (hereinafter referred to as bisphenol FL) with a carbonate precursor substance (U.S. Pat. No. 3,546,165). However, there are such problems as that when this aromatic polycarbonate is synthesized, a large amount of an insoluble gel is formed in the solvent and the yield of the solvent-soluble component is at most 60 to 70%, which hinder practicability; and that even when melt-molding of this aromatic polycarbonate is attempted, the melt viscosity is too high for it to be molded. Further, an alternating copolymer of bisphenol AF and bisphenol FL is also known (Macromolecules, Vol. 3, No. 5, 1970, 536 to 544). However, this copolymer is not such a random copolymer as described in the present invention, but an alternating copolymer in which the molar ratio of bisphenol AF and bisphenol FL is 50:50. The softening point of the alternating copolymer is too high compared with the random copolymer having specific molar ratio of the present invention, resulting it being difficult to be melt molded. Further, this alternating copolymer is poor in oxidation resistance and lacks in practicability.
It is also known that, when 1,1-bis(4-hydroxyphenyl)-1-phenylethane (hereinafter abbreviated to bisphenol AP) is reacted with a carbonate precursor substance, an aromatic polycarbonate high in glass transition temperature is obtained (Japanese Patent Application (OPI) No. 8317/1985). However, the polycarbonate from bisphenol AP is inferior to the polycarbonate from bisphenol A in heat stability.
As described above, since polycarbonates obtained from bisphenol A, which are conventionally known as typical aromatic polycarbonates, and which are obtained by reacting bisphenol A with phosgene are, transparent, recently their transparency has been taken advantage of and they have been developed for application in the field of information disks, optical fibers, lenses, etc.
Usually plastic optical fibers are high in light transmission loss and thus generally cannot be used for long-distance transmission, but since they are flexible and are easy in terminal workability, they should be useful for signal transmission lines of automobiles and electronic equipment. Since the core part of most conventional plastic optical fibers is made of a polymethyl methacrylate, it has a heat resistance no higher than 100.degree. C., and therefore the conventional plastic optical fibers cannot be used in engine compartments of automobiles or contained in heat-resistant parts of electronic equipment.
To improve this, in a case wherein heat resistance is needed, plastic optical fibers having a core part that uses a polycarbonate A (having repeating units with the below-given structural formula (A)) are used, but even the heat resistance of optical plastic fiber using this polycarbonate is only 125.degree. to 130.degree. C.
It is known that by using a polycarbonate AF (having repeating units with the below-given structural formula (B)) for a core part, a plastic optical fiber that can be used at a temperature of about 145.degree. C. can be obtained (Japanese Patent Application (OPI) Nos. 292105/1986 and 19307/1989).
However, polycarbonates AF are susceptible to hydrolysis and are poor in reliability under high temperatures, and when they are heated under a high humidity for a long period of time, the transmission loss increases. When a plastic optical fiber having a core part made of a polycarbonate AF is exposed to high temperatures, the elongation at breakage of the fiber lowers considerably. The glass transition temperature of polycarbonates AF is on the order of 160.degree. C., and the upper limit of the temperature at which the plastic optical fiber having a core part made of a polycarbonate AF can be used is about 145.degree. C.
Although improving the heat resistance of plastic optical fibers has been studied by using, as a core material, modified polycarbonates having higher glass transition temperatures, the moldability, the oxidation resistance, and the chemical stability are insufficient, and satisfactory results have not necessarily been obtained. For example, polycarbonates AP (having the below-given structural formula (C) and a glass transition temperature of 179.degree. C.), polycarbonates PP (having the below-given structural formula (D) and a glass transition temperature of 196.degree. C.), and polycarbonates Z (having the below-given structural formula (E) and a glass transition temperature of 170.degree. C.), are high in glass transition temperature, but they are poor in oxidation resistance and chemical stability, and their coloring advances in a short period of time under high temperatures, thereby causing an increase in transmission loss.
A plastic optical fiber was intended to be manufactured by using polycarbonates having the below-given structural formula (F), but molding was impossible because the molecule was rigid.
Further, a plastic optical fiber was intended to be manufactured by using polycarbonates having the below-given structural formula (G) and a glass transition temperature of 278.degree. C., but the molding of the polycarbonates into a plastic optical fiber was difficult because the melt-molding temperature was too high. ##STR1##